Crab and Lobster Fisheries in Scotland: Results of Stock ... 0814_0.pdf · inappropriate. Brown crab tagging studies are currently underway in Orkney and these may shed light on potential

Crab and Lobster Fisheries in Scotland: Results of

Stock Assessments 2013-2015

Scottish Marine and Freshwater Science Vol 8 No 14

C Mesquita, T Miethe, H Dobby and A McLay

Crab and Lobster Fisheries in Scotland: Results of Stock

Assessments 2013 - 2015

Scottish Marine and Freshwater Science Vol 08 No 14

Carlos Mesquita, Tanja Miethe, Helen Dobby and Anne McLay

Published by Marine Scotland Science

ISSN: 2043-7722

DOI: 10.7489/1990-1

Marine Scotland is the directorate of the Scottish Government responsible for

the integrated management of Scotland’s seas. Marine Scotland Science

(formerly Fisheries Research Services) provides expert scientific and technical

advice on marine and fisheries issues. Scottish Marine and Freshwater

Science is a series of reports that publishes the results of marine and

freshwater scientific work that has been carried out for Marine Scotland under

external commission. These reports are not subject to formal external peer-

review.

This report presents the results of marine and freshwater scientific work

carried out by Marine Scotland Science.

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1

Contents Page

EXECUTIVE SUMMARY ................................................................................................................................... 3

1. INTRODUCTION ........................................................................................................................................... 6

1.1. OVERVIEW OF SCOTLAND’S CRAB AND LOBSTER FISHERIES ....................................................................... 6

1.2. MANAGEMENT AND REGULATIONS ............................................................................................................. 7

2. DATA COLLECTION AND METHODS ........................................................................................................ 9

2.1. ASSESSMENT AREAS ................................................................................................................................ 9

2.2. LANDINGS DATA........................................................................................................................................ 9

2.3. CATCH-AT-LENGTH DATA .......................................................................................................................... 9

2.3.1. Landings-at-length .......................................................................................................................... 9

2.3.2. Discards ........................................................................................................................................ 10

2.3.3. Raising and Data Quality .............................................................................................................. 10

2.4. BIOLOGICAL DATA ................................................................................................................................... 11

2.5. SIZE-BASED INDICATORS AND SEX RATIO ................................................................................................. 11

2.6. LENGTH COHORT ANALYSIS (LCA) .......................................................................................................... 13

2.7. REFERENCE POINTS ............................................................................................................................... 14

3. RESULTS AND DISCUSSION BY SPECIES ............................................................................................. 15

3.1 BROWN CRAB .......................................................................................................................................... 15

3.1.1. The Fishery ................................................................................................................................... 15

3.1.2. Sampling Levels ............................................................................................................................ 15

3.1.3. Size-based Indicators and Sex Ratio ............................................................................................ 16

3.1.4. Stock Assessment ........................................................................................................................ 17

3.1.5. Comparison with Previous Assessments ...................................................................................... 18

3.1.6. Management Considerations ........................................................................................................ 18

3.2 VELVET CRAB .......................................................................................................................................... 18

3.2.1. The Fishery ................................................................................................................................... 18

3.2.2. Sampling Levels ............................................................................................................................ 19

3.2.3. Size indicators and Sex Ratio ....................................................................................................... 19




3.3 LOBSTER ................................................................................................................................................. 21

3.3.1. The Fishery ................................................................................................................................... 21

3.3.2. Sampling Levels ............................................................................................................................ 22

3.3.3. Size-based indicators and Sex Ratio ............................................................................................ 22




4. GENERAL DISCUSSION ............................................................................................................................ 25

4.1. LANDINGS ............................................................................................................................................... 25

4.2. STOCK ASSESSMENT .............................................................................................................................. 26

4.2.1. Brown Crab ................................................................................................................................... 26

4.2.2. Velvet Crab ................................................................................................................................... 27

2

4.2.3. Lobster .......................................................................................................................................... 27

4.3. QUALITY OF THE ASSESSMENT AND DATA ................................................................................................ 29

4.3.1. Landings Data ............................................................................................................................... 29

4.3.2. Biological Parameters ................................................................................................................... 29

4.3.3. Size-based Indicators ................................................................................................................... 31

4.3.4. Reference Points .......................................................................................................................... 32

4.4. DATA GAPS AND FUTURE RESEARCH PRIORITIES ..................................................................................... 33

4.4.1. Fishing Effort ................................................................................................................................. 33

4.4.2. Discard data .................................................................................................................................. 34

4.4.3. Population Structure ..................................................................................................................... 34

4.4.4. Growth studies .............................................................................................................................. 35

4.4.5. Other Factors ................................................................................................................................ 35

5. REFERENCES ............................................................................................................................................ 37

6. TABLES ...................................................................................................................................................... 41

7. FIGURES ..................................................................................................................................................... 49

8. ANNEXES ................................................................................................................................................... 78

8.1. ANNEX A: SAMPLING DATA – DECISIONS ON WHICH SPECIES/AREAS STOCK ASSESSMENTS WERE RUN ..... 78

8.2. ANNEX B: LENGTH FREQUENCY DISTRIBUTIONS ...................................................................................... 80

8.3. ANNEX C: BROWN CRAB AND VELVET CRAB PER RECRUIT ANALYSIS USING DIFFERENT BIOLOGICAL

PARAMETERS ................................................................................................................................................ 83

8.4. ANNEX D: ASSESSMENT AREAS NAMES AND ABBREVIATIONS ................................................................... 87

3

Executive Summary This report presents the results of Scottish regional brown crab (Cancer pagurus), velvet

crab (Necora puber) and lobster (Homarus gammarus) stock assessments carried out by

Marine Scotland Science (MSS) based on length cohort analyses (LCAs) applied to

commercial length frequency data for the period 2013-15.

The Fisheries

The Scottish creel fisheries are long established and tend to be mixed species fisheries

with brown crab, velvet crab and lobster as the main target species. The importance of

each species varies regionally and in relation to season and market demand. The

landings of the combined crab and lobster fishery into Scotland were 13,600 tonnes in

2015 with a first-sale value of over £29M.

Brown crab:

The most important crab species, in terms of weight and value, landed in Scotland is the

brown crab which is found all around the Scottish coast. In recent years, reported landings

of brown crab have fluctuated around 11,000 tonnes. The principal fishing areas for brown

crab in Scotland are the Hebrides, Orkney, Sule, East Coast, Papa and South Minch;

landings from these areas accounted for around 80% of the total in recent years.

Velvet crab:

Velvet crabs are caught in the inshore creel fishery with lobster and brown crab with few

fishermen fishing solely for velvet crab. In recent years, landings have decreased and

were around 1,500 tonnes in 2015. Most landings were taken from inshore areas.

Lobster:

The total tonnage of lobster landed in Scotland has always been much less than that of

crab but the average value per tonne is much higher. Lobster landings have increased

substantially over the last 15 years to about 1,000 tonnes in 2015. The majority of

landings of lobster in Scotland have been from the Hebrides, Orkney, South Minch, with

the South East and East Coast areas becoming increasingly important in more recent

years.

4

Assessments and Stock Status

LCA is the method used for assessing Scottish crab and lobster stocks. It uses

commercial catch size composition data and estimates of growth parameters and natural

mortality to estimate fishing mortality at length. Assessments are conducted on a regional

basis and males and females are assessed separately.

For the period between 2013 and 2015, LCAs were conducted for each species with

sufficient data, by assessment area. A summary of stock status in terms of fishing

mortality in relation to FMSY (FMAX was used as a proxy for FMSY) is provided for each stock.

Brown crab:

Brown crab fishing mortality for both males and females was estimated to be above FMSY

in the Clyde, East Coast, North Coast, Orkney, South East, South Minch and Sule areas.

In the Hebrides, fishing mortality for males was at FMSY while females were fished above

FMSY. In Papa, recent fishing mortality was around FMSY or lower. In Shetland, fishing

mortality for males was estimated to be above FMSY while assessment results for Shetland

females were inconclusive. No assessments were performed for the Mallaig and Ullapool

areas.

Velvet crab:

Velvet crab in the Clyde, East Coast, Orkney and South Minch, were estimated to be

fished at levels above FMSY (both males and females). In the Hebrides, recent fishing

mortality for males was below FMSY while females were fished above FMSY. In the South

East, males were fished at FMSY while females were fished above FMSY. Shetland

assessments were inconclusive and no assessments were performed for the Mallaig,

North Coast, Papa, Sule and Ullapool areas.

Lobster:

Fishing mortality was estimated to be above FMSY for both males and females in the Clyde,

East Coast, South East, Shetland and South Minch areas. In the Hebrides, Orkney and

Papa, the fishing mortality estimated for females was at or below FMSY while males were

fished above FMSY. No assessments were performed for the Mallaig, North Coast, Sule

and Ullapool areas as the sampling data collected were considered insufficient to run

LCAs.

Temporal trends in landings were explored using several size-based indicators for

the period over which sampling data are available for each species, by sex and

assessment area. The size at first capture, the mean size in the landings of

individuals above the size at first capture and the mean size of the largest 5% of

5

individuals were compared to the respective reference points. In several of well

sampled areas it was possible to relate variations of the mean size and mean size

of largest animals to trends in fishing mortality. In most cases results were in

agreement with the LCA results and showed similar exploitation level relative to

FMSY reference points.

Management Considerations

The results of assessments for the period 2013-15 indicated that in the majority of

the assessment areas, brown crab, velvet crab and lobster were fished close to or

above FMSY. In many of the assessment areas, a higher yield and biomass per

recruit in the long term could potentially be obtained by reducing the level of fishing

mortality (effort).

Data and Recommendations

With the exception of Shetland, limited effort data (pots fished) has been collected

from creel fisheries although recent changes in the FISH1 form may improve data

availability for vessels under 10 m. For larger offshore vessels, VMS data integrated

with logbook landings could potentially be used to obtain indicators of landings-per-

unit-effort and provide information on stock dynamics.

Discards in crab and lobster fisheries are sampled only on an irregular basis. More

regular sampling and information on catches of undersized animals could provide

an indication of inter-annual variation in recruitment.

The population structure and some aspects of the biology of crab and lobster are

not well understood and some of the assessment areas as currently defined may be

inappropriate. Brown crab tagging studies are currently underway in Orkney and

these may shed light on potential links between the offshore stock at Sule and

inshore Orkney stock.

Given the sensitivity of LCAs to the input growth parameters, further work in this

area is required. Field studies based on tagging methods and subsequent

evaluation of parameters would be desirable.

Information on factors affecting catchability such as bait type, creel density and

soak time could be collected by engaging with fishers and industry. Work being

conducted within the SIFIDS projects may provide an improved understanding of

the drivers of fishing behaviour.

6

1. Introduction

1.1. Overview of Scotland’s Crab and Lobster Fisheries

Crabs and lobsters are important species for the Scottish fishing industry. Although the

total quantity landed is small relative to finfish landings, crab and lobster attain high prices.

In 2015, the combined crab and lobster species landings of 16,000 tonnes had a value at

first-sale of over £29M (Scottish Government, 2016). The fishery is long established and

was traditionally an inshore mixed species creel fishery, prosecuted by small vessels.

However, improved technology and the ability to store and transport live animals in the

1980s led to the development of an offshore fishery for brown crab. This, and the demand

from new markets, has resulted in a substantial increase in the Scottish landings over the

last thirty years.

Creel fishing in Scotland continues today as an inshore fishery around most of the Scottish

coast, with vessels setting their gear within a few miles of the shore. The fishery is

typically a mixed species fishery with both crab and lobster being targeted, albeit with

some seasonal and regional variation. Most creel vessels are small, less than 10 m in

length and with only one or two crew, and make short day trips to haul creels. However, a

number of larger vessels now take part in an offshore fishery to the north and west of

Scotland. These vessels, which are up to 18 m in length and have a crew of four or five,

mainly target brown crab and carry vivier ponds on board to keep the catch alive. The size

and power of these vessels enables fishing on offshore grounds such as the Papa and

Sule Banks, with boats landing catch into ports such as Scrabster and Ullapool on a

weekly basis. These ‘vivier vessels’ or ‘super-crabbers’ are fewer in number than the

small day vessels but their catches represent over 50% of brown crab landings in some

regions. Local processors were the preferred market until the overseas market emerged

in the 1990s. By this time, technology had developed to enable the transport of live

animals by vivier lorries (specially adapted lorries containing cooled ponds for the transport

of live shellfish), within a day of landing. Today it is common for Scottish crabs and

lobsters to be transported live to the continent, particularly Spain, where there is a good

market. More recently, brown crab have also been exported to emergent Asian markets

including China, where “dry transport” of live animals is the preferred method.

The most important crab species, in terms of weight and value, landed in Scotland is the

edible or brown crab (Cancer pagurus) which is found all around the Scottish coast. The

second most important by landed weight is the velvet swimmer crab (Necora puber).

Velvet crabs are often caught in the inshore creel fishery with lobster and brown crab and

were once considered to be a ‘pest’ species. Very few fishermen fish solely for velvet

crab, although some target the species at certain times of the year. The Scottish fishery

7

expanded in the early 1980s following the collapse of the Spanish fishery (MacMullen,

1983), to become the largest in Europe (Tallack, 2002). Other crabs landed include deep-

water crabs (Chaceon sp.), the northern stone crab (Lithodes maja) and the shore or

green crab (Carcinus maenas), but these species only comprise a small proportion of the

total landings (Table 1).

The lobster most commonly landed in Scotland is the European lobster (Homarus

gammarus), a valuable species for which seasonal prices can be as high as £20 per kg at

first sale. Other lobsters landed include the spiny lobster or crawfish (Palinurus elephas),

squat lobsters (family: Galatheidae) and the Norway lobster (Nephrops norvegicus), also

known as langoustine or Nephrops. In Scottish waters P. elephas is caught mainly on the

west coast and along the north coast to Orkney and Shetland. Landings of P. elephas into

Scotland up to 1998 were consistently over 30 tonnes dropping to an average of around 7

tonnes over the last 10 years. Nephrops stocks are assessed by ICES (International

Council for the Exploration of the Sea) working groups and managed under the EU Total

Allowable Catch (TAC) system and are not considered further in this report.

1.2. Management and Regulations

In Scotland, vessels fishing for brown crab, velvet crab, spider crab, green crab, lobster or

crawfish must have a licence with a shellfish entitlement. The quantities that are permitted

to be landed are not restricted. Owners of vessels of length 10 m and under with a

shellfish entitlement are required to complete the FISH1 form for all landings of lobsters

and crabs and submit it on a weekly basis to the Fishery Office at which the vessel is

administered. In July 2016, the Scottish Government introduced a number of changes to

reporting on the FISH1 form as part of vessel licensing requirements. Fishermen are

required to report (1) the position where fishing activity starts on any fishing day (for creel

vessels this corresponds to the first position where creels are hauled); and (2) the number

of creels hauled associated with the declared landings quantity. For vessels of length over

10 m, data on fishing activity by trip must be recorded in a EU logbook and submitted to

the Fishery Office within 48 hours of landing. Licensed fishing vessels which do not hold a

shellfish entitlement are allowed to land a maximum of 5 lobsters and 25 crabs per day.

New regulations, effective from April 2017, restrict the numbers of certain shellfish species

that can be taken by unlicensed fishing boats on a daily basis. The restrictions are set

daily per vessel as, 1 lobster, 5 crabs (any species), 10 Nephrops and 6 scallops (The

Shellfish (Restrictions on Taking by Unlicensed Fishing Boats) (Scotland) Order, 2017).

There is currently a restrictive licensing system, whereby no new licenses and entitlements

are being granted. There are, however, non-active (latent) licence entitlements which

mean that there is the potential for the number of vessels fishing for crab and lobster to

8

increase. In 2014, participants in the European project ACRUNET (A Transnational

Approach to Competitiveness and Innovation in the Brown Crab Industry) produced an

analysis of latent capacity and the implications for the management of the brown crab

fisheries. It became clear that in the UK and Ireland there is significant latent capacity

(part of the fleet that is currently inactive but continues to hold fishing entitlements) that

could potentially target brown crab, but the likelihood of currently unused shellfish

entitlements becoming active could not be estimated at the time (Mesquita et al., 2015).

Crab and lobster fisheries are not subject to EU TAC regulations or national quotas,

although there are EU measures to restrict fishing effort. Under EU Regulations, the

annual fishing effort of UK vessels over 15 m participating in the brown crab fishery is

restricted to 702,292 KW days in ICES Areas 5 and 6 and 543,366 KW days in ICES Area

7 (EC, 2004).

Minimum landing size (MLS) regulations designed to protect juvenile animals apply to the

main commercial crab and lobster species. These are summarised in Table 2. In 2017

Marine Scotland will introduce a number of management measures. These include:

increases in MLS in some areas (Table 2), the prohibition of landing berried velvet crab

and a decrease in the maximum landing size of lobster (from 155 mm to 145 mm CL,

except Orkney and Shetland). Other extant regulations in the crab and lobster fishery are

described in detail in previous assessment reports (Mill et al., 2009; Mesquita et al., 2011).

This report presents summaries of historical and recent landings data and the results of

stock assessments for brown crab, velvet crab and lobster, updated with data collected

between 2013 and 2015. A description of the biology, habitat and life history of these

species is provided in Mill et al. (2009).

9

2. Data Collection and Methods

2.1. Assessment Areas

For assessment purposes, the Scottish creel fishing grounds are divided into 12

assessment areas as shown in Figure 1. These areas are based on the historical system

for reporting Scottish landings data (Thomas, 1958), subsequently revised to include two

offshore areas – Papa, which lies to the west of Shetland, and Sule, which is to the north

and west of Orkney and includes Rona, Sulisker and Sule-Skerry banks. Some Scottish

assessment areas extend outside Scottish Territorial Waters. On the east of Scotland the

South East assessment area extends beyond the Scottish border, while on the west coast,

the Clyde assessment area stops short of the Irish border. There is some fishing on

grounds outside the assessment areas. Currently these areas support only small fisheries

and landings data are monitored for any change in importance.

2.2. Landings Data

The assessments use official landings data, which provide location of capture (ICES

rectangle), the species and the weight landed into ports in Scotland. These data are

collated by Marine Scotland Compliance from sales notes and EU logbook and FISH1

forms, and are held in the Marine Scotland Fisheries Information Network (FIN) database

and in the Marine Scotland Science (MSS) Fisheries Management Database (FMD). Data

for brown crab landings from the Republic of Ireland (collected by the Irish Sea-Fisheries

Protection Authority – SFPA) were compiled and provided by the Irish Marine Institute.

These data were not used in the assessments but have been included to illustrate brown

crab landings by nation on a statistical rectangle basis to the west coast of Scotland.

2.3. Catch-at-length Data

2.3.1. Landings-at-length

Landings length-frequency data were collected by MSS as part of its market sampling

programme. Historical Shetland sampling data were provided by the North Atlantic

Fisheries College (NAFC) Marine Centre with the permission of the Shetland Shellfish

Management Organisation. Since 2010, data from fisheries in Shetland have been

collected and provided by staff at the NAFC Marine Centre under the Memorandum of

Understanding (MoU) between NAFC Marine Centre and MSS. From 2012, landings

length-frequency data collected by Orkney Sustainable Fisheries (OSF), mostly from the

Orkney assessment area (but also from Papa and Sule), have been shared with MSS.

10

These additional data have been used in combination with data collected by MSS from the

same areas. All the sampling data are held in the MSS Fisheries Management Database

(FMD).

Sampling measurements are taken as carapace length (CL) for lobsters, measured from

the eye-socket to the centre of the base of the thorax carapace, and carapace width (CW)

for brown and velvet crabs, measured across the widest part of the body, not including any

spines.

In general, sampling effort is focused in those areas where fisheries are most important.

However, the timing of landings is rather unpredictable and sampling is to some extent

opportunistic, factors which explain the variability in numbers sampled and the occurrence

of zeros for certain species in some areas. Brown crabs and lobsters may be retained for

a period of time in holding tanks after being ‘landed’ which makes them easier to access

for sampling. In contrast, velvet crabs are often landed in remote harbours and promptly

dispatched to fishing processors or shipped abroad. This makes it more difficult to get

samples, particularly in the Inner Hebrides (South Minch) in the west of Scotland.

MSS aims to conduct crab and lobster stock assessments every three years using length-

frequency data from the most recent three years. The stock assessments reported here

use sampling data collected between 2013 and 2015. The numbers of animals measured,

number of trips and percentage of sampled fishing trips (quotient between the number of

trips sampled and the total number of trips extracted from FIN) by assessment area are

shown, respectively, for each species in Table 3, Table 4 and Table 5.

2.3.2. Discards

Discard data are not regularly collected for the crab and lobster fisheries in Scotland, and

any mortality due to discarding practices is not taken into account in these assessments.

Anecdotal information and recent ad hoc unpublished studies suggest that crab and

lobster discard rates in the target creel fisheries are variable and occasionally high (>50%

by number). Discards typically comprise those animals that do not meet landing

regulations such as undersized individuals, post-moult animals with soft shells, v-notched

female lobsters or berried female crabs. It is thought that survival rates are likely to be

high and for assessment purposes landings are assumed equal to catch.

2.3.3. Raising and Data Quality

Length frequency data obtained from market sampling and official landings data were

combined to provide a raised annual landings-at-length distribution. Data were averaged

11

over a number of years (2013-15) and aggregated into 5 mm size classes for brown crab

and lobster and 3 mm size classes for velvet crab for use in the Length Cohort Analysis

(LCA).

Landings-at-length data were not available for the three species in all assessment areas.

The decision-making process to select which areas had sufficient data to run stock

assessments is presented in Annex A. Four parameters were used to categorise the

quality of the sampling data: number of trips/animals sampled; number of years for which

data are available (maximum three years – 2013-15); sampling seasonality; the shape of

the length frequency distribution (LFD). For each species/area combination, these

parameters were classified in one of three categories (“poor”/”ok”/”good”). Stock

assessments were not run for areas where one or more of the parameters was classified

as “poor” (see Annex A).

2.4. Biological Data

Information about the growth of British crabs and lobsters used in the LCA comes mainly

from tagging studies carried out in the 1960s and 70s (Hancock and Edwards, 1966;

Hancock and Edwards, 1967) (Table 6). Estimates of the von Bertalanffy growth

parameters: asymptotic length (L∞) and instantaneous growth rate (K) have been

estimated using Ford-Walford plots (Chapman, 1994; Tallack, 2002; Mouat et al., 2006).

Length-weight relationships (parameters a and b shown in Table 6) are from MSS

(unpublished) market sampling measurements of length and weight. Different, area

specific, biological parameters were applied in Shetland assessments as these are

available from research on these species conducted in Shetland (see Leslie et al., 2010 for

data sources).

2.5. Size-based Indicators and Sex Ratio

In theory, size-based indicators can provide information about the effects of fishing

pressure on a stock. Fishing mortality affects the abundance of populations but also their

age and size distribution. As cohorts age and grow, the effects of mortality accumulate

over time. With increasing mortality, fewer larger individuals can be expected in a stock

and associated catches. Life history processes such as maturation and fecundity depend

on size, such that the size structure of the stock affects the reproductive potential and

capacity for recovery (Trippel, 1998; Moland et al., 2010).

In this report, several size-based indicators were explored for the period over which

sampling data are available (brown crab and lobster – from 1981; velvet crab – from 1987)

for each species, by sex and assessment area. Any years in which the number of animals

12

sampled was less than 50 and species data from assessment areas sampled only once

(Sule for lobster, Mallaig for brown crab, and North Coast, Papa, Sule for velvet crab) were

excluded.

Size at first capture (Lc), the mean size in the landings of individuals above Lc, and the

mean size of the largest 5% of individuals were examined and plotted. The size at first

capture, Lc, was calculated as the size at half the maximum frequency in the ascending

part of a size frequency distribution. Ideally, the size at first capture, Lc, should be above

the size at first maturity, Lmat, to allow for reproduction before being caught by the fishery

(Myers and Mertz, 1998).

As a reference point for the mean size, we plotted the FMSY proxy LF=M (the expected

length in the catch when fishing mortality is equal to natural mortality). The proxy depends

on the ratio of natural mortality M and the growth rate K from the von Bertalanffy equation.

A ratio of 1.5 is assumed in standard data-limited cases where K and M are uncertain

(Prince et al., 2015). Other crab and lobster species have been characterized as having

an M/K ratio around 1.5, with unexploited stocks dominated in numbers by juveniles

(Prince et al., 2015). Species with lower M/K ratios, such as Nephrops, exhibit an adult

modal size (Prince et al., 2015). However, the parameter values for M and K as used in

the LCA (Table 6) were not recent, originating from Chapman (1994). As natural mortality

is difficult to estimate we use the default ratio of 1.5 for the calculation of reference point

LF=M , such that LF=M =0.75Lc+0.25L∞ (ICES, 2014b). L∞ refers to the asymptotic size

parameter from the von Bertalanffy growth equation (ICES, 2014b).

A decrease in mean size may be due to a reduction in the number of large individuals in

the population due to fishing pressure but may also be as a result of good recruitment,

which causes an increase in the number of small individuals. To mitigate the effects of

variable recruitment, the mean size of the largest 5% of landed individuals was examined

(ICES, 2014b; Probst et al., 2013). The mean size of the largest 5% was compared to the

reference point 0.9 L∞ which is considered to be a better FMSY proxy than the precautionary

reference point of 0.95 L∞ (Miethe and Dobby, 2016). A declining trend in the mean size of

largest animals can be associated with a reduction in the abundance of large individuals

due to the effects of fishing mortality. Due to data-limitation, we used available values of

life history parameters derived for particular regions such as Shetland, Hebrides and Firth

of Forth for other areas (Table 6,

Table 7).

Trends in the average annual sex ratio in the sampled landings were explored for each of

the three species, again excluding years in which fewer than 50 animals were sampled.

13

2.6. Length Cohort Analysis (LCA)

Age determination is generally not possible for animals that moult, and therefore, the

application of age-structured assessment methods to crustacean stocks is problematic

(Smith and Addison, 2003). Length Cohort Analysis (LCA) (Jones, 1984) is a commonly

used method of assessing stocks for which commercial catch length frequency distribution

data are available. LCA was used by MSS in previous assessments of the Scottish crab

and lobster stocks (Mill et al., 2009; Mesquita et al., 2011; Mesquita et al., 2016), by the

NAFC Marine Centre to assess crustacean stocks around Shetland (Leslie et al., 2007;

Leslie et al., 2010) and also by Cefas (Centre for Environment, Fisheries and Aquaculture

Science) to assess crab and lobster stocks in England (CEFAS, 2014a; CEFAS, 2014b).

The LCA method uses the commercial catch size composition data (length frequency data)

and estimates of growth parameters and natural mortality to estimate fishing mortality at

length. The key assumption of the approach is that the length distribution is representative

of a typical cohort over its lifespan. However, this is only true of length frequency data

from a single year if the population is in equilibrium, therefore, LCA is usually applied to

data averaged over a number of years during which recruitment and exploitation rates

have been stable. LCA also assumes uniform growth among animals. The results of LCA

can be used to predict changes in the long-term (equilibrium) stock biomass and yield-per-

recruit based on changes in mortality, potentially resulting from fishing effort reductions or

changes to minimum landing size regulations. The approach gives an indication of the

exploitation of the stock in terms of growth overfishing, but not recruitment overfishing.

LCAs were applied to all stocks where sampling data were collected and considered to be

sufficient (Annex A). To account for the differences in growth and length-weight

relationships (Table 6), males and females were assessed separately. For the Shetland

stocks of brown crab and velvet crab, two assessments were conducted using the two

alternative sets of biological parameters (Table 6). The LCA provides estimates of fishing

mortality (F) for each length class which are averaged over a fixed length range for each

species and sex to give an estimate of average fishing mortality (Fbar) for each stock.

Using a fixed length range in the calculation of Fbar enables comparisons of the recent F

value (2013-15) with those calculated in previous assessments (2002-05, 2006-08 and

2009-12) and offers the potential to detect trends in F.

14

2.7. Reference Points

The results of LCA were used to calculate yield-per-recruit (YPR) and biomass-per-recruit

(BPR) relative to changes in fishing mortality which provide an indication of stock status in

terms of growth overfishing. The relationship between YPR and F is typically dome

shaped – low levels of F result in low landings as few individuals are caught, whilst high

levels of F may also result in a reduction in yield (in addition to biomass) from a particular

cohort as animals are caught before they have had time to grow to a size that would

contribute much weight to the yield (growth overfishing). In between these lies FMAX, the

fishing mortality rate that maximizes YPR for a particular selection pattern. For data-

limited stock such as crabs and lobsters it is not possible to directly estimate the maximum

sustainable yield (MSY) and hence, in line with previous assessments, FMAX was used as a

proxy for FMSY. This approach has been widely used by ICES (ICES, 2010) for other

crustaceans such as Nephrops (e.g. ICES, 2014a). FMSY proxy values were taken from a

per-recruit analysis from an LCA of 2006–2008 landings-at-length data. Where 2006-08

sampling data were not available, data from the following round of assessments (2009-12)

were used instead. All FMSY proxy values remain preliminary and may be modified

following further data exploration and analysis. A summary of stock status in terms of

fishing mortality in relation to FMSY was provided for each stock. A stock was classified as

being fished “at FMSY” when the estimated F was within 10% of FMSY.

15

3. Results and Discussion by Species

3.1 Brown Crab

3.1.1. The Fishery

The brown crab fishery is long established and landings, although variable, have increased

significantly over the last 40 years (Figure 2a). Reported landings averaged around 2,000

tonnes in the late 1970s, increased to a maximum of about 12,000 tonnes in 2007 and

have fluctuated in recent years at around 11,000 tonnes. It is difficult to establish how

accurate the pre 2006 data are as landings are thought to have been under-recorded

before the introduction of the ‘buyers and sellers’ legislation. The value of landings has

increased in line with the tonnage landed. However, the price per kilogram has changed

little over this time (Figure 2b and Figure 2c). One kilogram of brown crab was sold for an

average of £1.28 at first sale during the period 2013 to 2015.

The annual brown crab landings by assessment area are shown in Figure 3. The principal

fishing areas for brown crab in Scotland are the Hebrides, Orkney, Sule, East Coast, Papa

and South Minch; landings from these areas accounted for around 80% of the total in

recent years (Table 8). Landings from the offshore areas of Sule and Papa increased

sharply in the 1990s when the fishery expanded, but seem to have stabilized in Papa and

decreased in Sule in the last three years. Landings from Orkney, East Coast and

Hebrides show an increasing trend in recent years. The spatial distribution of brown crab

landings by ICES statistical rectangle (including Irish landings) from 2013 to 2015 is

shown in Figure 6. There were no major changes in relation to the most important

rectangles for brown crab landings, compared to the previous assessments. Landings by

non-UK vessels taken from the Scottish assessment areas were relatively low (mostly by

Irish vessels) and usually confined to the South Minch (rectangle 40E3). The Irish fishery

in Scottish waters has contracted in recent years with grounds in the Hebrides, Sule and

North Coast not currently fished by Irish vessels. There is an important fishery for brown

crab on the Malin shelf (Irish assessment area) that is exploited by Irish vessels, with most

landings being taken from the Donegal region (rectangles 40E2, 39E2 and 39E1). There

were no other regions of importance for brown crab landings around Scotland outside the

Scottish assessment areas.

3.1.2. Sampling Levels

The number of sampled brown crabs, number of trips and percentage of sampled fishing

trips are shown in Table 3. Good sampling coverage was achieved for the most important

(in terms of landings) assessment areas, with samples being obtained throughout the year

16

in the period 2013-15. A decline in the number of sampling trips was noted in Sule from

2014 with no samples collected in 2015. The percentage of trips sampled was generally

less than 3% in assessment areas where daily inshore trips are common. In the Orkney

(where OSF has been sharing considerable amounts of data with MSS) and Papa

assessment areas which include offshore grounds, sampling percentages were higher

(Table 3) as the fishery is dominated by larger vessels which tend to make fewer but

longer trips. Sampling data from the Clyde (which was not included in the previous round

of assessments) for 2013-15 was sufficient to conduct an assessment. Sampling data

were considered to be insufficient (low numbers and infrequent sampling) to run

assessments in the Mallaig and Ullapool areas (Annex A). Amongst the assessed areas,

the South Minch had the lowest sampling levels with less than 1% of fishing trips sampled

in the period 2013-15. However, data included samples from all quarters and the

averaged length frequency (Annex B) were similar to those in other adjacent areas and

therefore, assumed to be representative, despite the low sampling levels.

3.1.3. Size-based Indicators and Sex Ratio

Size-based indicators of brown crab in the landings from each assessment area are shown

for males in Figure 9 and for females in Figure 10. The data are typically noisy. The mean

sizes of brown crabs caught on the west coast are generally larger than in other areas. In

recent years (2013-15), the mean size of males was above the respective reference point

in the Hebrides and Papa (Table 14). For females, the mean size was above the

reference point only in Orkney. In several areas, the mean size in the landings appears to

have increased (both sexes), which is driven mainly by an increase in length at first

capture following the implementation of a MLS in 1998.

In recent years (2013-15), the mean sizes of the largest 5% of males and females were

above the reference point in Papa (Table 14). There is some evidence of a positive trend

in the mean size of the largest individuals throughout the time series in Shetland and the

Clyde. In Shetland, the mean size of the largest females was close to the reference point

in 2015. In contrast, a negative trend was observed in the mean size of the largest

individuals since early 2000s for both sexes in the East Coast, South East and Sule.

The sex ratio in landings of brown crab varied greatly between assessment areas but in

most cases shows no trend over time (Figure 15). Consistently higher percentages of

females in landings over the time series are evident in areas which include offshore fishing

grounds such as the Hebrides (88%), Papa (87%), Sule (85%) and North Coast (82%).

The only inshore area with a clear predominance of females in landings is the South Minch

(68%). The South East is a male dominated fishery (70% males). Orkney has shown a

decrease in the male percentage in landings from over 70% in the 1980s to around 50% in

17

recent years, whereas the sex ratio in Shetland and East Coast displayed the opposite

trend.

3.1.4. Stock Assessment

Results of assessments based on LCAs and per recruit analysis, summarising estimates

of fishing mortality in relation to FMSY, are shown below. Estimated fishing mortalities in

relation to previous assessments are presented in Figure 18 (males) and Figure 19

(females). Brown crab biomass and yield-per-recruit plots for each assessment area are

shown in Figure 24 (males) and Figure 25 (females).

Brown crab stock status, relationship between F and FMSY for 2006-08, 2009-12 and 2013-15.

F (Fishing Mortality)

Assessment

period

2006-

2008

2009-

2012 2013-15

Clyde Males Above FMSY

Females Above FMSY

Hebrides Males At FMSY

Females Above FMSY

North

Coast

Males Above FMSY

Females Above FMSY

Papa Males Below FMSY

Females At FMSY

Shetland Males Above FMSY

Females Unknown

Sule Males Above FMSY

Females Above FMSY


Assessment

period

2006-

2008

2009-

2012 2013-15

East

Coast

Males Above FMSY

Females Above FMSY

Mallaig Males Unknown

Females Unknown

Orkney Males Above FMSY

Females Above FMSY

South

East

Males Above FMSY

Females Above FMSY

South

Minch

Males Above FMSY

Females Above FMSY

Ullapool Males Unknown

Females Unknown

In the most recent assessments, nine of the ten assessed areas were fished above FMSY to

some extent (Table 11). Fishing mortality for both males and females was estimated to be

above FMSY in the Clyde, East Coast, North Coast, Orkney, South East, South Minch and

Sule areas. In the Hebrides, fishing mortality for males was at FMSY while females were

fished above FMSY. In Papa, recent fishing mortality was around FMSY or lower. In

Shetland, fishing mortality for males was estimated to be above FMSY while assessment

results for Shetland females were deemed inconclusive due to contradictory results

obtained when using the two alternate sets of biological parameters (see section below

and discussion). No assessments were performed for Mallaig and Ullapool areas as the

sampling data collected were considered insufficient to run LCAs.

18

3.1.5. Comparison with Previous Assessments

The current assessment uses a fixed length range to calculate an average fishing mortality

and can be compared with previous assessments which used the same range (Mill et al.,

2009; Mesquita et al., 2011; Mesquita et al., 2016). Estimated F has increased in relation

to the last assessment in the East Coast, Hebrides, North Coast, Orkney, South Minch and

Sule (males and females), South East, Shetland (males) and Papa (females). F has

shown a slight decreasing trend in the South East (females) and remained approximately

the same in Papa (males) and the Clyde (Figure 18 and Figure 19). The main change in

status in relation to FMSY are evident in the North Coast (both sexes) and Sule (males)

which were previously at or below FMSY and are now fished above FMSY.

Estimates of F for Shetland (using Shetland parameters) are higher than those estimated

elsewhere (Table 11, Figure 18 and Figure 19). This is due to the use of different growth

rate (K) and natural mortality (M) parameters specific to Shetland (Table 6). To examine

what underlies these differences, LCAs were run for all areas using both the Shetland and

the rest of Scotland biological parameters and results compared (Annex C). Shetland’s

higher natural mortality rate results in flat-topped YPR curves with both higher estimates of

current F and FMAX (the FMSY proxy) compared with other areas, particularly for female

brown crab (Figure 19 and Table 11). This is further discussed in section 4.3.

3.1.6. Management Considerations

The results of assessments for the period 2013-15 showed that brown crab in the majority

of the assessment areas in Scotland were fished close to or above FMSY. In many of the

assessment areas, a higher yield and biomass per recruit in the long term could potentially

be obtained by reducing the level of fishing mortality (effort).

3.2 Velvet Crab

3.2.1. The Fishery

Velvet crab landings increased gradually until the mid-1990s followed by a slight decline

up to 2005 and a sharp increase in 2006. It is not clear, however, whether this increase in

landings reflects the introduction of the ‘buyers and sellers’ legislation or an expansion of

the fishery at this time. In recent years, landings have decreased and were around 1,500

tonnes in 2015. The value per unit weight of velvet crab remains higher than that of brown

crab and is currently approximately £2.50 per kilogram (Figure 2b). The three areas that

have historically had significant velvet crab fisheries are the Hebrides, Orkney and South

Minch, although the fisheries in the two latter areas have shown a marked decrease over

19

the last ten years (Figure 4). These three areas accounted for about 65% of velvet crab

landings in Scotland in the period 2013-15 (Table 9). Figure 7 shows the spatial

distribution of velvet crab landings 2013-15. Most landings were taken from inshore areas;

only very small amounts were reported from offshore grounds in Papa and Sule. There

were no significant landings of velvet crab from around Scotland reported from outside the

assessment areas.


The numbers of sampled velvet crabs, number of trips and percentage of sampled fishing

trips are shown in Table 4. The percentage of trips sampled was generally lower than that

achieved for other species. In the Clyde, Hebrides, Orkney and Shetland, temporal

coverage was good with samples generally being obtained throughout the year in the

period 2013-15. The East Coast and South East have smaller velvet crab fisheries and

generally lower sampling rates than the main areas. The South Minch is one of the

important areas for the velvet crab fishery where the number of samples has been

relatively low although this improved in 2013-15 compared to 2009-12, when data were not

collected in all years (Mesquita et al., 2016). It has proved difficult to access and measure

velvet crabs caught and landed in the Inner Hebrides which explains the low rate of trips

sampled in the South Minch. Assessments were conducted for the Clyde, Hebrides,

Orkney and Shetland and also for South Minch, East Coast and South East (the latter was

not assessed in 2009-12). Sampling data were considered to be insufficient (low numbers

and infrequent sampling) to run assessments in the Mallaig North Coast, Papa, Sule and

Ullapool areas (Annex A).

3.2.3. Size Indicators and Sex Ratio

Size-based indicators for velvet crab by assessment area are shown for males in Figure

11 and for females in Figure 12. The time series of sampled landings is shorter than for

brown crab. The mean sizes of individuals in landings were relatively stable. In 2013-15,

the mean sizes of the largest 5% of both males and females were on average below the

reference points in all assessment areas (Table 15). However, there is a weak positive

trend in the mean size of the largest 5% since 2000 in the Hebrides and in Orkney and

since 2010 in the Clyde. A decrease in the mean size and the size of the largest 5% of

individuals was noted in the East Coast since 2008. The size at first capture was generally

above maturation size (Figure 11, Figure 12).

The sex ratio for velvet crab showed little evidence of a trend with males dominating in the

landings, representing 65% to 72% (by number) in the well sampled areas (Figure 16).

20


The results of assessments based on LCAs and per recruit analysis, summarising

estimates of fishing mortality in relation to FMSY are shown below. Estimates of fishing

mortality in relation to previous assessments are presented in Figure 20 (males) and

Figure 21 (females). Velvet crab biomass and yield-per-recruit plots for each assessment

area are shown in Figure 26 (males) and Figure 27 (females).

Velvet crab in the Clyde, East Coast, Orkney and South Minch, were fished at levels

above FMSY (both males and females) in the most recent assessments. In the Hebrides,

recent fishing mortality for males was estimated to be below FMSY while females were

fished above FMSY. In the South East, males were fished at FMSY while females were

fished above FMSY (Table 12). Assessment results for Shetland were deemed inconclusive

due to contradictory results in the estimated fishing mortality when using the two alternate

sets of biological parameters (see section below and discussion). No assessments were

performed for the Mallaig, North Coast, Papa, Sule and Ullapool as the sampling data

collected were considered insufficient to run LCAs.

Velvet crab stock status, relationship between F and FMSY for 2006-08, 2009-12 and 2013-15.


Assessment

period

2006-

2008

2009-

2012 2013-15


Females Above FMSY

Hebrides Males Below FMSY

Females Above FMSY

North

Coast

Males Unknown

Females Unknown

Papa Males Unknown

Females Unknown

Shetland Males Unknown

Females Unknown

Sule Males Unknown

Females Unknown


Assessment

period

2006-

2008

2009-

2012 2013-15

East

Coast

Males Above FMSY

Females Above FMSY


Females Unknown


Females Above FMSY

South

East

Males At FMSY

Females Above FMSY

South

Minch

Males Above FMSY

Females Above FMSY


Females Unknown

21




2009; Mesquita et al., 2011; Mesquita et al., 2016). In the South Minch, female fishing

mortality increased above FMSY in the most recent assessment while in the other areas the

position of F relative to FMSY remains unchanged. The estimated F for velvet crab has

been found to be relatively stable over the time series in the Hebrides and Orkney areas

(Figure 20 and Figure 21). Velvet crab F estimates for Shetland (using Shetland

parameters) are much higher than those estimated elsewhere (Table 12, Figure 20 and

Figure 21). This is due to the use of different growth rate (K) and natural mortality (M)

parameters specific to Shetland (Table 6). To examine what underlies these differences,

LCAs were run for all areas using both the Shetland and the rest of Scotland biological

parameters and results compared (Annex C). Shetland’s higher natural mortality rate

results in flat-topped YPR curves with both higher estimates of current F (Fmale=3.0;

Ffemale=3.4) and FMAX (the FMSY proxy) for both sexes compared with other areas (Figure

20, Figure 21 and Table 12). This is further discussed in section 4.3.


The results of assessments for the period 2013-15 showed that velvet crab in the majority

of the areas with sufficient sampling data to conduct assessments were fished close to or

above FMSY. In some assessment areas (Clyde, East Coast, Orkney and South Minch), a

higher yield and biomass per recruit in the long term could potentially be obtained by

reducing the level of fishing mortality (effort).

3.3 Lobster

3.3.1. The Fishery

The total tonnage of lobster landed in Scotland has consistently been much lower than that

of crabs. However, reported lobster landings have increased substantially over the last 15

years, from 415 tonnes in 2001 to about 1,000 tonnes in 2015 (Figure 2a). The average

price per kilogram of lobster has been stable at over £10 per kilogram over the last 15

years (Figure 2c), and is nine times higher than that of brown crab. Between 2013 and

2015 the total value of the lobster fishery was around 80% that of the brown crab fishery

(Figure 2b). The annual lobster landings by assessment area are shown in Figure 5.

Historically, the majority of landings of lobster in Scotland have been from the Hebrides,

Orkney and South Minch, with the South East and East Coast areas becoming

increasingly important in more recent years. The period between 1999 and 2004 was

22

characterised by lower landings from all areas. This can be related to an increase in

minimum landing size to 87 mm in 1999, with the effect on landings being evident for the

following years. Landings in 2006 do not seem to be comparable with those in the

preceding years (particularly in the South East and East Coast) which could be due to the

introduction of ‘buyers and sellers’ regulations, before which landings may have been

under-reported. Landings from the South East and the East Coast increased continuously

from 2006 to 2011 and accounted for almost 60% of landings into Scotland over the last

three years (Table 10). Figure 8 shows the spatial distribution of lobster landings around

Scotland in the period 2013-15. ICES rectangle 41E7 in the South East consistently has

the largest amount of landings. Small quantities of lobster were landed from grounds

outside the assessment areas, including ICES rectangles to the west of the South Minch,

to the south of the Clyde and just outside the South East and East Coast areas.


The numbers of sampled lobsters, number of trips and percentage of sampled fishing trips

are shown in Table 5. The percentage of trips sampled was generally less than 3% in

most assessment areas, where daily inshore trips are common. The best sampled areas

were the East Coast, South East, Hebrides, Orkney and Shetland. Despite an increase in

sampling in the South Minch and Papa in recent years, which allowed assessments to be

run in 2009-12, the number of sampling trips carried out in these areas remains lower than

elsewhere. Length frequencies derived for these areas typically show a similar distribution

to those from better sampled, adjacent areas and are therefore assumed to be

representative, despite the low sampling levels. Sampling data were considered to be

insufficient (low numbers and infrequent sampling) to run assessments in the Mallaig,

North Coast, Sule and Ullapool areas (Annex A).

3.3.3. Size-based Indicators and Sex Ratio

Size-based indicators for lobster landings by assessment area are shown in Figure 13 for

males and in Figure 14 for females. The data are very noisy, and although males and

females appeared to generally follow the same pattern of variation, there is little evidence

of trends in any of the areas. Lobsters in the South East and East Coast were noticeably

smaller than in other areas over the full time series. A decline in the mean size of larger

individuals can be observed in Shetland over the most recent 5 years for both sexes. For

males only, there is a slight decreasing trend in the East Coast over the past five years

and in the Hebrides over the last ten years. In recent years, the mean size and the mean

size of the largest 5% of males were generally below the reference points (Table 16). Only

in individual years for females in Papa (2013) and Hebrides (2013, 2015) were both the

recent mean size and the mean size of the largest 5% above the reference points. In the

23

East Coast and South East, the size at first capture was above the size at first maturity for

both sexes. In the other assessment areas, maturation size is assumed to be larger and

size at first capture was below size at maturity.

The sex ratio in landings for lobsters was close to 50% in all areas and showed no trends

(Figure 17).


The results of assessments based on LCAs and per recruit analysis summarising

estimates of fishing mortality in relation to FMSY are shown below. Estimates of fishing

mortality in relation to previous assessments are presented in Figure 22 (males) and

Figure 23 (females). Lobster biomass and yield-per-recruit plots for each assessment

area are shown in Figure 28 (males) and Figure 29 (females).

In the most recent assessments, lobsters in all the areas were fished above FMSY to some

extent, particularly males. Fishing mortality was estimated to be above FMSY for both

males and females in the Clyde, East Coast, South East, Shetland and South Minch. In

the Hebrides, Orkney and Papa, fishing mortality for females was at or below FMSY while

males were fished above FMSY (Table 13). No assessments were performed for Mallaig,

North Coast, Sule and Ullapool as the sampling data collected were considered insufficient

to run LCAs.

Lobster stock status, relationship between F and FMSY for 2006-08, 2009-12 and 2013-15. F (Fishing Mortality)

Assessment

period

2006-

2008

2009-

2012 2013-15


Females Above FMSY

Hebrides Males Above FMSY

Females Below FMSY

North

Coast

Males Unknown

Females Unknown

Papa Males Above FMSY

Females Below FMSY

Shetland Males Above FMSY

Females Above FMSY

Sule Males Unknown

Females Unknown


Assessment

period

2006-

2008

2009-

2012 2013-15

East

Coast

Males Above FMSY

Females Above FMSY


Females Unknown


Females At FMSY

South

East

Males Above FMSY

Females Above FMSY

South

Minch

Males Above FMSY

Females Above FMSY


Females Unknown

24




2009; Mesquita et al., 2011; Mesquita et al., 2016). Shetland’s lobster results are only

presented from 2009 onwards. The MoU between MSS and NAFC for data provision

means that recent data (sampled landings) are not directly comparable with data provided

prior to 2009 (raised catch data including discards).

In Shetland, estimated F for males increased and was above FMSY in the latest

assessment. An increase in F was also found for males in the East Coast and for females

in Shetland and the Clyde. The South East estimated F decreased in 2013-15 for males

and females but remains above FMSY. Estimates of F for female stocks were generally

lower than males, showing a slight decrease in relation to the previous assessments in the

Hebrides and Orkney. Estimates of F in Papa remained approximately the same in

relation to previous assessments. The estimated F for lobster has been found to be

relatively stable over the time series in the Hebrides area (Table 13, Figure 22 and Figure

23).


The results of assessments for the period 2013-15 indicated that lobster in the majority of

the assessment areas in Scotland were fished close to or above FMSY. In all assessment

areas, a higher yield and biomass per recruit in the long term could potentially be obtained

by reducing the level of fishing mortality (effort).

25

4. General Discussion

4.1. Landings

Brown crab remains the most important species, in terms of landed weight, in the crab and

lobster fisheries around Scotland. Landings from Orkney, East Coast and Hebrides

increased, whereas those from the offshore areas of Papa and Sule appear to have

stabilized or decreased in the last three years. The latter may be related to market prices,

which have not increased substantially in recent years, and reduced effort rather than any

change in the abundance of the stock offshore. Total landings of velvet crab show a

decreasing trend in recent years. There have been notable declines in reported landings

from traditionally important areas including Orkney and South Minch. The spatial

distribution of the velvet crab fishery is similar to previous years and the fishery continues

to take place in inshore areas. Lobster landings, although still much lower than those of

brown and velvet crabs, have more than doubled since 2001, with the East Coast and

South East areas making the major contribution to this increase.

Landings of all three species are thought to have been under-reported prior to 2006,

before the introduction of the ‘buyers and sellers’ legislation. Some major increases in

landings evident in the mid 2000s are more likely to be explained by improved reporting

than by changes in the abundance of stocks. There is little information on changes in

fishing effort over the past 30 years, but it is likely that technological advancement and

mechanisation of fishing and processing have allowed the crab and lobster fishery to

expand and effort to increase. The emergence of European markets, and more recently

eastern markets in China, combined with the ability to transport live animals has increased

the demand, particularly for brown crab. However, this has not been accompanied by an

increase in crab prices, particularly for brown crab for which the price per kilo is lower than

it was 10 years ago. Market and supply issues contributed to the setting up of a European

project ACRUNET (A Transnational Approach to Competitiveness and Innovation in the

Brown Crab Industry) in 2012. ACRUNET comprised several work packages with the

collective aim of securing the economic and social viability and sustainability of the

European brown crab industry. The project highlighted the disparity of fishery

management measures in relation to the latent capacity in the fishery, between the main

European crab producer countries, France on one hand and UK and Ireland on the other.

In recent years, both the UK and Ireland fishermen have emphasized the issue of latent

effort, pointing out that this was one of the biggest obstacles to shellfish management.

The latent effort was quantified as part of the ACRUNET project (Mesquita et al., 2015)

and this information is likely to be useful in future discussions regarding the management

of brown crab fisheries.

26

4.2. Stock Assessment

4.2.1. Brown Crab

Brown crab sampling levels were relatively high in most areas where landings are

important. In the northwest of Scotland, female crabs usually make the largest

contribution to the landings. This suggests that females are exposed to higher exploitation

than males on some of the offshore grounds such as the Hebrides or Papa and, in

general, this is supported by the LCA results. Tagging studies suggests that females

migrate from deeper offshore grounds, where they live for most of the year, to inshore

areas where they moult and mate with males (Edwards, 1979; Jones et al., 2010). Large

females also predominate in the landings from offshore areas to the north of Ireland (Tully

et al., 2006), and in the fisheries off the east coast of England and in the English Channel,

aggregations of ovigerous females have been observed (e.g. Howard, 1982).

The evaluation of size-based indicators relative to reference points, allows for some

inferences to be made on stock status in terms of exploitation level. In recent years, both

mean size and mean size of the largest 5% were above the reference points for females in

Papa and for males in Hebrides and Papa. This suggests that despite high exploitation in

Papa both sexes appear to have extended size structure. The mean size of the largest

individuals in Shetland increased throughout the time series, in particular for females. In

contrast, the mean size of the largest individuals decreased in the East Coast, South East

and Sule in recent years, particularly for males. This could be indicative of a decrease in

the proportion of large individuals in the stock, and possibly an increase in fishing

mortality, especially for Sule. It could, however, also be a reflection of changes in fishing

or discard practices. Without further fishery information it is difficult to conclude which is

more likely.

Assessments based on the 2013-15 data (using LCA) were carried out for brown crab in

ten of the twelve assessment areas. Fishing mortality for stocks in most of the areas

remains above FMSY. For those stocks, a long term reduction in effort would probably

increase yield and biomass-per-recruit. The results from the LCA were in general

agreement with the indicator evaluation in relation to reference points. In the Papa (males

and females) and Hebrides (males) areas both approaches suggested that these stocks

were being fished below the reference points. The mean sizes of brown crabs in the

South East and East Coast assessment areas were smaller than in other areas, as found

in previous assessments (e.g., Kinnear, 1988; Mill et al., 2009). It should be noted that the

growth parameters used in the assessments are the same for all areas and that this may

not be appropriate if individuals in the East and South East are slower growing and reach

smaller maximum sizes. Use of inappropriate growth parameters in the LCA could lead to

27

overestimation of F and the conclusion that the stocks are more heavily fished than they

actually are. This is discussed further in section 4.3 below. Assessment results using

2013-15 data for the North Coast, Hebrides, Sule, Orkney and East Coast all show a clear

increase in fishing mortality estimates compared to those reported previously (Mesquita et

al., 2016), with most crab stocks fished well above FMSY.

4.2.2. Velvet Crab

Male velvet crabs were more common in the landings than females and were generally

slightly larger. With respect to mean size and mean size of the largest 5% of individuals in

2013-15, all stocks of velvet crabs appeared to have truncated size distributions and are

exploited above the FMSY proxy. Only the mean size of males in the Hebrides indicated

exploitation close to FMSY (mean size just below LF=M) in the most recent year. If the

landings LFDs are representative of the population size structure and there have been no

recent changes in the exploitation pattern, the increase in mean size and mean size of the

largest individuals observed in the Hebrides would indicate that the fishing mortality may

have declined over the time series in this area. Decreasing trends in the mean size and

the mean size of the largest individuals were observed in the East Coast which may

indicate (assuming no changes in the fishery) an increase in fishing mortality. These

areas also show sporadic occurrences of large numbers of small individuals in the length

frequency data, which could be due to increased recruitment in these years.

Assessments based on the 2013-15 data (using LCA) were carried out for velvet crab in

six of the twelve assessment areas. Fishing mortality for stocks in most of these areas

remains above FMSY. For those stocks, a long term reduction in effort would probably

increase yield and biomass-per-recruit. For both Orkney and male Hebrides stocks,

fishing mortality showed a decreasing trend from the 2002-05 assessment estimate (Mill et

al., 2009), although in 2013-15 Orkney remained above FMSY while Hebrides male velvet

crabs were fished below FMSY and females above FMSY. Fishing mortality in the South

Minch female stock has increased compared to previous assessments and is now above

FMSY. Results based on indicators for velvet crabs were less optimistic than the LCA

analysis. However, the mean size of the largest males in Hebrides increased in recent

years, such that, if this continued, the reference point may be reached in the coming years.

4.2.3. Lobster

Lobster market sampling data from 2013-15 suggest that male and female lobsters were

generally landed in equal proportions. For most areas, the mean size has remained stable

in recent years. Following the evaluation of size-based indicators relative to the reference

points, males appear to be exploited above the FMSY proxy (mean size less than LF=M) in all

28

areas. In recent years the mean size in landings of the largest males has decreased in the

Hebrides and East Coast, which may indicate an increase in the long term fishing mortality

although this could also be a reflection of changes in the fishing practices. For females,

indicators were generally below the reference points in 2013-15, with exception of mean

size in Hebrides. In some individual years, the mean size of the largest females in Papa

(2013) and the Hebrides (2013, 2015) were above the size reference point. According to

the indicators, female lobster in Orkney were still slightly below their size reference point.

In the South East and Orkney, the mean sizes, mean sizes of the largest individuals and

length frequency data appear relatively stable suggesting a stable stock and fishery. The

size at 50% maturity can vary substantially between assessment areas. Currently, the

evaluation of exploitation level on immature individuals should be viewed as preliminary as

length at maturity is not known for all areas.

The evaluation of mean size and mean size of largest 5% in the landings relative to the

reference points was generally in agreement with the LCA. It is assumed that lobsters to

the west of Scotland as well as at Orkney and Shetland mature at a larger size than to the

east of Scotland. Length at first catch was above length at first maturity only in the East

Coast and South East.

Assessments based on the 2013-15 data (using LCA) were carried out for lobster in eight

of the twelve assessment areas. Fishing mortality for stocks in most of the areas remains

above FMSY, especially among the males. For those stocks, a long term reduction in effort

would probably increase yield and biomass-per-recruit. The results from the LCA were in

general agreement with the indicator evaluation. Females in Papa, Hebrides and Orkney

were fished below or at FMSY while in other areas F is above FMSY. There are differences

in mean size in the sampled landings between areas, with lobsters from the North Coast

and Hebrides being significantly larger than those from the South East and East Coast

areas. It should be noted that the growth parameters assumed are the same for all areas,

except Shetland. This may not be appropriate if individual growth is generally slower in

some areas such the East Coast and South East, and individuals typically attain smaller

maximum sizes. The use of inappropriate growth parameters in the LCA could

overestimate the degree of growth overfishing. This is discussed further in section 4.3.

29

4.3. Quality of the Assessment and Data

4.3.1. Landings Data

From the range of stock assessment tools available, LCA is one of the least data intensive,

and LCA and yield-per-recruit models are commonly used for assessing data-poor

shellfish stocks. A major assumption of LCA is that the landings length frequency

distribution is representative of the fishery removals from a single cohort of individuals

throughout its life. However, since the length frequencies are derived from a single year of

sampling, rather than from a single cohort, this assumption is only true if the population is

in a steady state or at equilibrium, i.e. that recruitment and exploitation rate are constant.

Landings from most of the Scottish assessment areas tend to fluctuate, which may reflect

year to year variation in recruitment and/or fishing effort. An average of the length

frequency data (2013-15) was used in order to limit the effects of these variations.

However, any systematic changes in exploitation rate or recruitment over this three year

period could still result in biased estimates of fishing mortality.

Landings are generally well sampled, for length and sex composition in the most important

fishing areas. However, in some areas such as South Minch (brown crab and lobster), the

Clyde (brown crab) and Papa (lobster), size data are sparse, and sampling levels remain

relatively low with few sampling trips taking place. Length frequencies derived for these

areas are often similar to those in other adjacent areas and are therefore assumed to be

representative, despite the low sampling levels. However, the effect of this assumption on

LCAs has not been investigated and the results of these assessments should be

interpreted with caution.

4.3.2. Biological Parameters

LCA is frequently used for assessing crustacean species for which ageing techniques are

not yet fully developed. Discontinuous growth within a cohort of animals (growth

increments and frequency of moults) results in growth rates varying between individuals

within a stock. This can make it difficult to track cohorts through length frequency data and

hence assessment methods which translate between size-structured and age-structured

data have not been widely applied to crustaceans (e.g. Sheehy et al., 1996; Sheehy and

Prior, 2008; Kilada, 2012; Kilada and Acuña, 2015).

In addition to landings length frequency distribution data, LCA also requires estimates of

other biological parameters, including von Bertalanffy growth parameters and natural

mortality. LCA is very sensitive to these parameters and the choice of input parameters

may critically influence the results obtained (Lai and Gallucci, 1988; Jones, 1990), such as

30

the perception of the state of the stock, in terms of the position of the current exploitation

rate in relation to FMSY. Natural mortality (M), for example, has a marked effect on the

shape of the relative yield per recruit curve. Using lower values for M results in a more

pessimistic stock assessment, with current fishing mortality estimated to be higher in

relation to FMSY (or vice-versa). The values of the von Bertalanffy growth parameters (K

and L∞) also affect the shape of the yield per recruit curve and estimation of the value of F

in relation to FMSY. Using growth model parameters that result in growth rates that exceed

the true growth rate (i.e. using values of K and L∞ which are too large), results in the

current exploitation rate (F) being over-estimated in LCA and could lead to the erroneous

conclusion that a stock is growth-overfished (and vice-versa).

In these assessments, the same biological parameters have been applied across all areas

except Shetland (Chapman, 1994; Mill et al., 2009; Mesquita et al., 2011; Mesquita et al.,

2016). Studies of velvet crabs in Shetland (Tallack, 2002; Mouat et al., 2006) provided

much higher estimates of M (0.58 compared to 0.1 elsewhere) and a higher value of K

(0.46 compared to 0.1 elsewhere). For brown crab in Shetland, M is estimated to be

higher than elsewhere (0.25 compared to 0.1) and the sex-specific von Bertalanffy K

parameters are also different (lower at Shetland for males, but higher for females). The

high value of K estimated for Shetland velvet crabs implies a very fast growth rate for

young crabs (for example a 30 mm male crab would be expected to grow to nearly 60 mm

carapace length in a single year). This seems unrealistic and merits closer scrutiny of the

original data and methods used to estimate the growth parameters for this stock. For

brown crab, the parameters derived from studies in Shetland are closer to those used by

MSS for other assessment areas, but some notable differences persist, in particular for

females. The difference in parameters is sufficient to explain the disparity in the results

obtained for the velvet and brown crab assessments in Shetland when compared to other

stocks with relatively similar length frequency distributions. Velvets in the Clyde, East

Coast, Orkney and South Minch are estimated to be growth overfished, whereas in

Shetland they are estimated to be fished below FMAX (as estimated from a LCA using

Shetland parameters), despite the estimated F in Shetland being much higher than in

other areas. The F calculated from a LCA applied to Shetland data using the rest of

Scotland parameters is much lower but estimated to be above FMAX. For brown crab, as M

and K parameters from Shetland and elsewhere are closer, F estimates are similar but the

estimated FMAX, taken as a proxy for FMSY, from Shetland parameters is always higher.

This reflects the shape of the YPR curves generated in the LCAs using Shetland

parameters, which are more flat topped resulting in a higher FMAX to be taken as a

reference point. Because of the differences in the estimates of F obtained using the

different sets of growth parameters and the associated issues of interpretation for the

purposes of this report, we have described the stock status for Shetland velvet crab and

brown crab as unknown. Owing to the uncertainty around appropriate input parameter

31

values, care is required in drawing firm conclusions regarding the status of the crab and

lobster stocks, especially for velvet crab. To progress this discussion it would be

worthwhile holding a joint NAFC Shetland/MSS crab working group in the near future.

Differences in size composition across areas, particularly the relatively small size of brown

crab and lobster landed in the South East and East Coast compared to the north and west,

suggest that area specific parameter values may be more appropriate and it is possible

that the extent of growth overfishing of brown crab and lobster in the East Coast and South

East is overestimated. Estimation of growth parameters for these areas would, however,

require a large scale tagging project using tags that could be reliably retained on moult,

with seasonal measurements of length and weight.

4.3.3. Size-based Indicators

ICES has previously suggested a multiple-indicator-based approach (including LPUE,

size-based indicators and recruitment indices) as a potential way forward in the provision

of advice on stock status for crab stocks (ICES, 2009). An approach using commercial

length frequency data and biological parameters in the evaluation of stock status for data

limited stocks has recently been implemented by ICES and is followed here (e.g. ICES,

2015a; Miethe et al., 2016). In the assessments presented here, size-based indicators

were calculated and compared to the respective reference points. In some cases it was

possible to relate variations of the mean size and mean size of largest animals to trends in

fishing mortality. The results are highly dependent on the quality of estimates of life history

parameters by assessment area. In most cases results were in agreement with the LCA

results and indicated similar exploitation levels relative to FMSY reference points.

Mean sizes of the largest individuals are quite variable from year to year, particularly in

areas where data collection is more sporadic. This is likely to reflect sampling variability

rather than changes in the population. The sampling levels achieved in some areas

remain low. Improved sampling and better information on fishing activity and fishers’

behaviour can help to develop robust size-based indicators for assessment purposes.

Assumptions for biological parameters (M/K, L∞, Lmat) are also important for the

interpretation of size-based indicators. The estimated mean size indicator reference point

LF=M depends on the ratio between natural mortality and the growth parameters. The

collection of data on growth rate in different areas, in order to derive area specific

parameters, would help to improve the assessment using size-based indicators. With the

exception of Shetland, the M/K ratios used for the LCA were below 1.

32

A lower M/K ratio would result in a higher, more restrictive, size-based reference point,

relating to an expectation of more large individuals in the size distribution of an unexploited

stock (Hordyk et al., 2015; Jardim et al., 2015).

4.3.4. Reference Points

LCA provides long term equilibrium predictions and assumes constant recruitment and

exploitation rates. It is therefore advisable to complement the outputs with additional data

which can provide information on trends in abundance, typically catch per unit effort data

or exploitation rate. Effort data in terms of numbers of creels fished are not currently

available for Scottish creel fisheries, precluding calculation of catch per unit effort. In an

attempt to gain additional information on variation of fishing mortality from the available

data , the mean size and in the mean size of largest 5% of individuals were explored

(ICES, 2014b; ICES, 2015a).

The conclusions in this report are all based on estimates of fishing mortality in relation to a

reference point for each stock to infer whether or not a stock is fished above the level that

would result in MSY. For the purposes of consistency in this report, all discussion relates

to the FMSY proxy (FMAX). Although LCA and yield-per-recruit analysis give an indication of

current F relative to the fishing mortality required to optimise yield (from a particular

cohort), it provides no indication of whether or not a stock is recruitment overfished (i.e.

whether fishing is compromising recruitment). FMSY, the fishing mortality which gives the

maximum sustainable yield (high long term yield with low risk of stock depletion), is difficult

to estimate. In addition to a yield-per-recruit curve, calculating FMSY requires good

estimates of recruitment and spawning stock biomass, which are not available for Scottish

crab and lobster stocks. In cases where FMSY cannot be estimated directly, proxy values

based on yield per recruit analysis are often used. ICES advises that in cases where the

peak in the yield per recruit curve is well defined and there is no evidence of poor

recruitment at this level of fishing mortality, then FMAX may be an appropriate proxy for

FMSY (ICES, 2010). In cases where the peak is less well defined and the curve is more flat

topped, then F0.1 is likely to be a more appropriate proxy (Jennings et al., 2001). Another

potential reference point is F30%SpR which is defined as the fishing rate which results in

combined spawning biomass per recruit equal to 30% of the un-fished level. F0.1 is usually

the most conservative reference point while FMAX is generally above F30%SpR, depending on

the relative shape of the YPR and BPR curves. FMSY proxies for Nephrops stocks

assessed by ICES have been selected from these three candidate reference points (F0.1,

F35%SpR or FMAX) for each stock independently according to the perception of stock

resilience, typical population density, biological knowledge and the nature of the fishery

(e.g. ICES, 2015b). Most crab and lobster fisheries have been in existence for several

decades with little evidence of between stock differences in resilience.

33

Therefore, despite some stocks showing very flat topped YPR curves (which might

suggest F0.1 as the most appropriate proxy for FMSY), FMAX was selected as a proxy for

FMSY for all stocks (Mesquita et al., 2016). However, these reference points remain

preliminary and may be revised in the future as further data become available.

In most areas around Scotland, the crab and lobster stocks are being fished at levels

which result in yield per recruit values not far below the maximum. However, in some

cases, the estimated fishing mortality is substantially above FMSY, making it more likely that

these stocks are recruitment overfished as well as growth overfished. It should be noted

however, that, so far, lobster stocks have not showed signals of systematic changes in sex

ratio which has been associated with recruitment overfishing in other lobster species.

4.4. Data Gaps and Future Research Priorities

From the discussion above it is clear that there are a number of areas where research or

additional data collection would improve Scotland’s crab and lobster stock assessments.

4.4.1. Fishing Effort

Prior to 2016, no useful measures of creel fishing effort are available from official log

sheets with the exception of the Shetland area where the Shetland Regulating Order

requires licensed fishers to return logbook information to the SSMO, detailing the catch

location (at the 5 nmi scale) and the number of creels or pots fished. This has precluded

the use of LPUE data as an indicator of abundance for the crab and lobster stocks around

Scotland. Fishing effort for most finfish species can be estimated as fishing time (days

fishing or KW days) using days absent from port and vessel power but these are not

particularly useful measures of effort in creel fisheries. The number of creels used or

hauled when fishing for crabs and lobsters is considered to be a much more useful

measure of effort in the fishery. The recent changes to reporting on the FISH1 form

include the introduction of a mandatory field for the number of creels hauled. This will

provide new effort data for vessels under 10 m fishing around Scotland and should allow

for calculation of LPUE indices in the future.

VMS (vessel monitoring system) data have become available for larger vessels (over 15 m

from 2008 and over 12 m from 2012), and could potentially be integrated with logbook

landings information to obtain indicators of LPUE for the offshore fleets. However, these

monitoring tools do not cover the majority of the inshore fleet which is mostly composed of

smaller vessels (under 10 m). A number of work packages within the Scottish Inshore

Fisheries Integrated Data System project (SIFIDS) (recently funded by the European

Maritime Fisheries Fund, EMFF), are concerned with improving data collection from

34

inshore creel vessels through the use of new technology. It is envisaged that one of the

project outcomes will be a system that can be used on small vessels to collect and

transmit spatial fishery data including effort. Data collection would, however, need to be

coordinated and maintained, to build up useful time series.

4.4.2. Discard Data

Discards in crab and lobster fisheries are not sampled on a regular basis. More regular

sampling to obtain information on catches of undersized animals could provide an

indication of inter-annual variation in recruitment. By-catch data collected on MSS scallop

surveys may potentially provide information on variation in recruitment in areas where the

scallop and the crab and lobster distributions overlap. Further discard studies are also

required to obtain estimates of discard survival and to help understand more fully the

reasons for discarding in crab and lobster fisheries.

The results of a series of pilot projects to support sustainable Scottish inshore fisheries,

funded by the European Fisheries Fund suggest that many of these data deficiencies

could be addressed through self-sampling and EM technology (Course et al., 2015).

Further advances in data collection technology are expected as part of the SIFIDS project

(see Section 4.4.1).

4.4.3. Population Structure

The population structure of crab and lobster stocks around Scotland, and the rest of the

UK, is not well understood. The current assessment areas are empirical, based largely on

past fishing patterns. Brown crab are known to undertake extensive seasonal migrations

in some areas while in contrast, velvet crabs and lobsters appear to make relatively limited

movements. MSS previously conducted a tagging study of brown crab to the north of

Scotland (Jones et al., 2010). The results suggest linkage between inshore and offshore

crab stocks to the north and west of Scotland. Fishermen support the idea that crabs

migrate between (and across) the ‘windsock’ and inshore grounds around Orkney,

although there is only a limited fishery in the area in between. Large catches of female

crab have also been reported on the shelf edge at depths greater than 200 m. Work being

undertaken in both Shetland and Orkney, should provide further evidence regarding the

structure of brown crab stocks to the north of Scotland. Ideally, such studies should be

followed up by population genetics/morphology studies and consideration of larval

dispersal.

35

It has been suggested, for example, that brown crab populations in the Irish Sea may be

closely linked to the larger populations on the Malin shelf, which are contiguous with the

Hebrides and South Minch in western Scotland (Tully et al., 2006; ICES, 2007). The

current brown crab assessment areas will be reviewed by ICES WGCRAB in the future.

4.4.4. Growth Studies

The currently used growth parameter estimates for crustaceans in Scotland were derived

from tagging studies which took place in the 70’s in a few selected areas. More recent

studies were carried out in Shetland and these show some relevant differences to those

values previously estimated, in particular for the parameter K in crabs. Given the

sensitivity of LCAs to the input growth parameters, further work in this area is required,

especially for velvet crabs for which available data suggest very different growth rates

between Shetland and elsewhere. Field studies based on tagging methods (using tags

retained on moult) and subsequent evaluation of parameters would be desirable.

4.4.5. Other Factors

The interpretation of trends in indicators derived from fishery dependent data would be

helped by improved understanding of the economic and environmental factors that

influence fishers’ decision making with respect to fishing location and target species. A

component of the Lot 1 EU project (Anon, 2010) involved conducting questionnaires and

interviews to establish the main factors in fishers’ decision making. As well as providing

information on historical changes in fishing practices, the interview responses suggested

that the Scottish brown crab fishery has been influenced more by the market than by

management measures. Many fishers acknowledged that low brown crab market prices in

the late 2000s resulted in the species being targeted to a lesser extent than usual, which

could explain the reduction in landings in some areas. Additional information on factors

affecting catchability such as bait type, creel density and soak time could also be collected

by engaging with fishers and industry. Through liaison with fishers and the use of spatial

data on fishing location, work being conducted within the SIFIDS project ought to provide

an improved understanding of the drivers of fishing behaviour.

36

Acknowledgements

The authors would like to express their thanks to staff from Marine Scotland Science and

Marine Scotland Compliance staff collected and collated the data used in this report,

NAFC Marine Centre and Orkney Sustainable Fisheries Ltd for providing size frequency

data for the Shetland and Orkney areas, and the Marine Institute for providing brown crab

landings from the Republic of Ireland in ICES Subarea 6.

37

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viability in wild European lobster Homarus gammarus. Marine Ecology Progress Series,

400: 165-173.

Mouat, B., Laurenson, C., Riley, D., Marrs, S., Henderson, S. Shetland Shellfish Stock Assessments

2006. Shetland Shellfish Management Organisation: North Atlantic Fisheries College,

Marine Centre; 2006

Myers, R.A., Mertz, G., 1998. The limits of exploitation: A precautionary approach. Ecological

Applications, 8: S165-S169.

Prince, J., Hordyk, A., Valencia, S.R., Loneragan, N., Sainsbury, K., 2015. Revisiting the concept

of Beverton - Holt life-history invariants with the aim of informing data-poor fisheries

assessment. Ices Journal of Marine Science, 72: 194-203.

Probst, W.N., Kloppmann, M., Kraus, G., 2013. Indicator-based status assessment of commercial

fish species in the North Sea according to the EU Marine Strategy Framework Directive

(MSFD). ICES Journal of Marine Science, 70: 694-706.

Scottish Government. 2016. Scottish Sea Fisheries Statistics 2016.

http://www.acrunet.eu/images/ACRUNET/Technical_Reports/Activity_3/ACRUNET_Technical_Report_Activity3_Action5.pdf%5d

http://www.acrunet.eu/images/ACRUNET/Technical_Reports/Activity_3/ACRUNET_Technical_Report_Activity3_Action5.pdf%5d

40

Sheehy, M.R.J., Prior, A.E., 2008. Progress on an old question for stock assessment of the edible

crab, Cancer pagurus. Marine Ecology-Progress Series, 353: 191-201.

Sheehy, M.R.J., Shelton, P.M.J., Wickins, J.F., Belchier, M., Gaten, E., 1996. Ageing the European

lobster Homarus gammarus by the lipofuscin in its eyestalk ganglia. Marine Ecology-

Progress Series, 143: 99-111.

Smith, M.T., Addison, J.T., 2003. Methods for stock assessment of crustacean fisheries. Fisheries

Research, 65: 231-256.

Tallack, M.S.L. The biology and exploitation of three crab species in the Shetland Islands,

Scotland.; 2002

The Lobsters and Crawfish (Prohibition of Fishing and Landing) (Scotland) Order. Scottish

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Thomas, H.J., 1958. Lobster and crab fisheries in Scotland. Marine Research, 8: 107.

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pagurus L.) Fishery: Analysis of the resource in 2004-2005. Fisheries Resource Series. Dun

Loaghaire, Ireland: Bord Iascaigh Mhara (Irish Sea Fisheries Board); 2006

41

6. Tables

Table 1 – Crab and lobster species landed into Scotland by UK vessels between 2009 and 2015. Landings information from the Fisheries Management Database.

Landings (tonnes)

2009 2010 2011 2012 2013 2014 2015

Brown crab 9465 10546 11832 10892 10891 12306 11089

Velvet crab 2728 2474 2147 2032 1576 1641 1494

Deep-water crab 11 6 8 0 2 16 2

Northern stone

crab

6 3 1 2 0 0 1

Green crab 226 214 237 301 272 225 194

European lobster 1092 1100 1219 1132 1026 1208 1042

Spiny lobster 4 6 5 13 9 5 4

Squat lobsters 2 1 1 1 1 1 1

Table 2 – Minimum landing size (MLS) regulations in Scotland for commercially important crab and lobster species (CW – carapace width mm; CL carapace length mm).

Species MLS

(mm) Area

Year measure

introduced

Brown

Crab1

150

(CW) Hebrides 2016

140

(CW)

All areas except Hebrides and East coast

South of 56° 1998

130

(CW)

East coast South of 56° excluding Firth of

Forth 1998

Lobster2 87 (CL) All areas except Shetland and Hebrides 1999

90 (CL) Shetland and Hebrides 2001/2016

Velvet

Crab3

65 (CW) All areas except Shetland and Hebrides 1989

70 (CW) Shetland and Hebrides 2001/2016

1 An increase in MLS to 150 mm is planned for most areas in 2017.

2 A phased increase to 90 mm in MLS is planned for some areas in 2017.

3 An increase in MLS to 70 mm is planned for all areas where MLS is currently 65 mm in 2017.

42

Table 3 – MSS market sampling statistics, number of animals measured, number of trips sampled and percentage of trips sampled for brown crab by assessment area, 2013-15.

Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Animals

measured

2013 343 4913 6052 0 1423 29648 4533 2096 5025 676 9365 194

2014 267 6292 8838 0 1030 24016 4705 2405 5238 379 1316 0

2015 418 6079 5892 0 1422 53372 3651 1185 4474 428 0 0

Trips

sampled

2013 7 25 30 0 3 137 4 17 45 9 9 2

2014 5 36 22 0 1 186 3 13 41 4 2 0

2015 6 32 24 0 2 385 2 8 43 4 0 0

Percentage

of trips

sampled (%)

2013 3.3 0.9 2.1 0 1.0 5.8 4.0 2 2.4 0.7 5.2 0.3

2014 2.1 1.1 2.1 0 0.4 7.5 2.0 1.4 2.0 0.2 1.0 0

2015 2.6 0.9 2.4 0 0.6 15.0 2.0 1.1 2.1 0.3 0 0

Table 4 – MSS market sampling statistics, number of animals measured, number of trips sampled and percentage of trips sampled for velvet crab by assessment area, 2013-15.

Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Animals

measured

2013 2914 1514 2848 0 0 4100 0 4427 509 1798 0 228

2014 3502 1177 2313 73 0 9852 0 4272 2113 649 0 0

2015 1870 745 2403 0 0 22116 0 3233 1657 1160 0 0

Trips

sampled

2013 15 10 17 0 0 17 0 9 5 11 0 2

2014 18 8 16 1 0 46 0 10 13 4 0 0

2015 10 4 14 0 0 94 0 9 15 6 0 0

Percentage

of trips

sampled (%)

2013 10.1 0.5 1.5 0 0 0.8 0 1.0 0.3 1.0 0 0.7

2014 9.0 0.4 1.5 2.3 0 2.1 0 1.3 0.8 0.3 0 0

2015 6.2 0.2 1.7 0 0 4.3 0 1.3 0.9 0.4 0 0

43

Table 5 – MSS market sampling statistics, number of animals measured, number of trips sampled and percentage of trips sampled for lobster by assessment area, 2013-15.

Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Animals

measured

2013 633 879 1018 0 51 634 209 2123 1629 116 0 85

2014 244 1608 2477 0 22 1860 265 1967 3475 103 0 0

2015 231 574 1832 0 362 6185 77 2338 3046 300 0 0

Trips

sampled

2013 13 17 25 0 1 17 2 14 26 6 0 1

2014 8 20 24 0 1 56 3 18 66 3 0 0

2015 8 11 23 0 2 173 2 10 43 6 0 0

Percentage

of trips

sampled (%)

2013 4.0 0.5 1.9 0 0.4 0.7 2.7 2.2 0.8 0.5 0 0.3

2014 1.9 0.5 2.0 0 0.4 2.2 2.4 3.0 1.9 0.2 0 0

2015 1.9 0.3 2.2 0 0.7 6.7 2.7 1.8 1.2 0.4 0 0

Table 6 – Biological parameters used in stock assessment for brown crab, velvet crab and lobster.

Growth

parameters

Length-Weight

relationship

Terminal

group

Fishing

Mortality

Natural

Mortality

Source

K L∞ a b F M

Brown crab

Males 0.197 220 0.000059 3.214 0.5 0.1 Chapman, 1994

Females 0.172 220 0.000302 2.8534 0.5 0.1 Chapman, 1994

Shetland Males 0.188 246 0.00008 3.166 0.406 0.242 Tallack, 2002

Shetland Females 0.224 227 0.00024 2.895 0.174 0.256 Tallack, 2002

Velvet crab

Males 0.105 103 0.0003 3.0389 1.9 0.1 Chapman, 1994

Females 0.118 100 0.0009 2.7405 1.1 0.1 Chapman, 1994

Shetland Males 0.463 107 0.0011 2.75 0.31 0.576 Tallack, 2002

Shetland Females 0.463 100.1 0.0038 2.42 0.202 0.576 Tallack, 2002

Lobster

Males 0.11 173.4 0.000126 3.36 0.5 0.1 Chapman, 1994

Females 0.13 150 0.000919 2.922 0.5 0.1 Chapman, 1994

Shetland Males 0.112 188 0.0017 2.797 0.316 0.1 Mouat et al., 2006

Shetland Females 0.136 184 0.0004 3.123 0.452 0.1 Mouat et al., 2006

44

Table 7 – Size at 50% maturity, Lmat (mm) for brown crab, velvet crab and lobster.

Source

Lmat

Brown crab Tallack,

2002

Males 125 (Shetland data)

Females 133.5

Velvet crab Tallack,

2002

Males 45 (Shetland data)

Females 56

Lobster Lizárraga-Cubedo et al.,

2003

East Coast

&

Males

80

(data from Firth of Forth)

South East Females 79

Other areas Males 98 (data from Hebrides)*

Females 110

* Potentially higher variability between assessment areas along the west coast.

45

Table 8 – Percentage of total landings into Scotland of brown crab 2013–2015, by assessment area. Data are from the Fisheries Management Database.

Species Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Brown crab (%)

2013 1.5 11.7 19.6 0.1 5.3 17.6 8.6 5.6 4.3 8.0 13.7 4.0

2014 1.5 10.6 21.7 0.1 4.4 16.0 10.1 5.4 3.2 9.7 13.9 3.4

2015 1.6 10.9 20.1 0.1 9.2 18.5 8.4 4.1 4.1 6.3 14.8 1.9

Table 9 – Percentage of total landings into Scotland of velvet crab 2013–2015, by assessment area. Data are from the Fisheries Management Database.

Species Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Velvet crab (%)

2013 4.3 10.8 16.5 0.4 0.5 31.9 0.1 11.7 7.4 14.7 0 1.7

2014 6.1 10.1 17.1 0.9 0.4 31.5 0 9.9 6.0 16.2 0 1.8

2015 5.8 10.0 17.8 1.4 0.5 30.8 0 9.2 5.4 18.1 0 1.0

Table 10 – Percentage of total landings into Scotland of lobster 2013–2015, by assessment area. Data are from the Fisheries Management Database.

Species Year

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Lobster (%)

2013 2.4 21.8 9.9 0.1 1.0 11.9 0.6 3.6 39.4 7.6 0.1 1.6

2014 3.9 19.3 12.7 0.1 0.9 14.0 0.7 3.4 34.9 8.6 0.1 1.4

2015 3.6 23.0 11.6 0.1 1.3 11.5 0.3 4.1 35.2 7.9 0.1 1.3

46

Table 11 – Brown crab fishing mortality (Fbar) from Length Cohort Analysis (2013-15) in relation to FMSY. Fishing mortality calculated across a fixed range of 145-190 mm CW interval (150-200 mm for Shetland).

Assessment

area

Fbar FMSY

FMSY basis

Males Females Males Females

Clyde 0.50 0.43 0.39 0.20 LCA 2006-08

East Coast 0.70 0.46 0.37 0.30 LCA 2006-08

Hebrides 0.42 0.51 0.41 0.35 LCA 2006-08

Mallaig - - - - -

North Coast 0.61 0.52 0.39 0.43 LCA 2006-08

Orkney 0.77 0.45 0.33 0.36 LCA 2006-08

Papa Bank 0.30 0.38 0.36 0.39 LCA 2009-12

Shetland 0.68 0.41 0.38 0.31 LCA 2013-15

Shetland par* 0.99 0.56 0.84 1.72 LCA 2013-15

South East 0.82 0.53 0.39 0.28 LCA 2006-08

South Minch 0.75 0.5 0.34 0.25 LCA 2006-08

Sule 0.64 0.52 0.36 0.36 LCA 2006-08

Ullapool - - - - -

*Fs calculated using Shetland biological parameters

Table 12 – Velvet crab fishing mortality (Fbar) from Length Cohort Analysis (2013-15) in relation to FMSY. Fishing mortality calculated across a fixed range of 60-80 mm CW interval (75-85 mm for Shetland).

Assessment

area

Fbar FMSY

FMSY basis


Clyde 0.38 0.55 0.15 0.39 LCA 2006-08

East Coast 0.35 0.54 0.15 0.21 LCA 2006-08

Hebrides 0.17 0.34 0.20 0.23 LCA 2006-08

Mallaig - - - - -

North Coast - - - - -

Orkney 0.21 0.38 0.08 0.17 LCA 2006-08

Papa Bank - - - - -

Shetland 0.60 1.03 0.48 0.58 LCA 2013-15

Shetland par* 3.04 3.43 3.89 4.70 LCA 2013-15

South East 0.22 0.47 0.22 0.26 LCA 2006-08

South Minch 0.31 0.53 0.26 0.43 LCA 2006-08

Sule - - - - -

Ullapool - - - - -

*Fs calculated using Shetland biological parameters

47

Table 13 – Lobster fishing mortality (Fbar) from Length Cohort Analysis (2013-15) in relation to FMSY. Fishing mortality calculated across a fixed range of 95-130 mm CL interval (110-155 mm for Shetland).

Assessment

area

Fbar FMSY

FMSY basis


Clyde 0.85 0.64 0.26 0.28 LCA 2006-08

East Coast 0.40 0.43 0.26 0.32 LCA 2006-08

Hebrides 0.33 0.27 0.18 0.34 LCA 2006-08

Mallaig - - - - -

North Coast - - - - -

Orkney 0.43 0.32 0.23 0.32 LCA 2006-08

Papa Bank 0.36 0.19 0.21 0.31 LCA 2009-12

Shetland 0.36 0.46 0.24 0.16 LCA 2009-12

South East 0.51 0.48 0.35 0.36 LCA 2006-08

South Minch 0.50 0.59 0.36 0.41 LCA 2009-12

Sule - - - - -

Ullapool - - - - -

Table 14 – Brown crab. Size-based indicators as a ratio between mean length, , and mean length of the largest , max5% and the respective reference points (average for 2013-15). Values larger than 1 (green) and equal to 1 (yellow) indicate exploitation consistent with FMSY.

Assessment

area

/ LF=M Lmax5%/0.9L∞


Clyde 1.00 1.00 1.00 0.96

East Coast 0.97 1.00 0.96 0.99

Hebrides 1.01 0.99 1.00 0.97

Mallaig - - - -

North Coast 0.99 0.99 0.97 0.97

Orkney 0.98 1.01 0.94 0.99

Papa Bank 1.02 1.00 1.06 1.01

Shetland 0.94 1.00 0.86 0.97

South East 0.97 0.99 0.94 0.97

South Minch 0.97 0.99 0.95 0.98

Sule* - - - -

Ullapool - - - -

*Average 2013-15 not calculated as 2015 data were missing.

48

Table 15 – Velvet crab. Size-based indicators as a ratio between mean length, , and mean length of the largest , max5% and the respective reference points (average for 2013-15).

Assessment

area

/LF=M Lmax5%/0.9L∞


Clyde 0.96 0.94 0.89 0.88

East Coast 0.96 0.95 0.90 0.85

Hebrides 0.99 0.97 0.95 0.92

Mallaig - - - -

North Coast - - - -

Orkney 0.98 0.97 0.93 0.92

Papa Bank - - - -

Shetland 0.95 0.95 0.89 0.91

South East 0.97 0.96 0.93 0.90

South Minch 0.96 0.94 0.90 0.89

Sule - - - -

Ullapool - - - -

Table 16 – Lobster. Size-based indicators as a ratio between mean length, , and mean length of the largest , max5% and the respective reference points (average for 2013-15). Values larger than 1 (green) and equal to 1 (yellow) indicate exploitation consistent with FMSY.

Assessment

area

/LF=M Lmax5%/0.9L∞


Clyde 0.89 0.92 0.76 0.84

East Coast 0.90 0.92 0.85 0.88

Hebrides 0.99 1.00 0.91 0.99

Mallaig - - - -

North Coast - - - -

Orkney 0.93 0.96 0.87 0.94

Papa Bank - - - -

Shetland 0.95 0.92 0.86 0.86

South East 0.89 0.91 0.81 0.84

South Minch 0.92 - 0.81 -

Sule - - - -

Ullapool - - - -

49

7. Figures

Figure 1. Crab and lobster fishery assessment areas in Scotland. See Annex D for

abbreviations.

50

Figure 2. Scottish crab and lobster fishery statistics. a) Landings (tonnes) into Scotland, b) landings value (£IM), and, c) price per kilo (£/kg) for brown crab, velvet crab and lobster, 1974 -2015. Data sourced from Fisheries Management Database.

Land

ings

(Ton

nes)

(a)

Land

ings

(Ton

nes)

0

2000

4000

6000

8000

10000

12000 Brow n crab Lobster Velvet crabVa

lue

(£)

0

5

10

15

(b)

Val

ue (£

1M)

Year

Pric

e pe

r kilo

(£/k

g)

0

2

4

6

8

10

12

1980 1990 2000 2010

(c)

Pric

e pe

r kilo

(£/k

g)

Year

51

Figure 3. Brown crab landings (tonnes) into Scotland by assessment area, 1974-2015.

Data from the Fisheries Management Database; ‘Outside’ relates to brown crab landed

outside MSS crab and lobster assessment areas; see Figure 1 for area locations and

Annex D for abbreviations.

CL

0

500

1000

1500

2000

2500EC HE

MA

0

500

1000

1500

2000

2500 NC OR

PA

0

500

1000

1500

2000

2500SH SE

SM

0

500

1000

1500

2000

2500 SU

1980 1990 2000 2010

UL

1980 1990 2000 2010'Outside'

1980 1990 2000 2010

0

500

1000

1500

2000

2500

Year

La

nd

ing

s (

To

nn

es)

52

Figure 4. Velvet crab landings (tonnes) into Scotland by assessment area, 1984-2015.

Data from the Fisheries Management Database; ‘Outside’ relates to velvet crab landed

outside MSS crab and lobster assessment areas; see Figure 1 for area locations and

Annex D for abbreviations.

CL

0

200

400

600

800

1000

1200 EC HE

MA

0

200

400

600

800

1000

1200 NC OR

PA

0

200

400

600

800

1000

1200 SH SE

SM

0

200

400

600

800

1000

1200 SU

1985 1990 1995 2000 2005 2010 2015

UL

1985 1990 1995 2000 2005 2010 2015'Outside'

1985 1990 1995 2000 2005 2010 2015

0

200

400

600

800

1000

1200

Year

La

nd

ing

s (

To

nn

es)

53

Figure 5. Lobster landings (tonnes) into Scotland by assessment area, 1974-2015. Data

from the Fisheries Management Database. ‘Outside’ relates to lobster landed outside

MSS crab and lobster assessment areas; see Figure 1 for area locations and Annex D for

abbreviations.

CL

0

100

200

300

400EC HE

MA

0

100

200

300

400NC OR

PA

0

100

200

300

400SH SE

SM

0

100

200

300

400SU

1980 1990 2000 2010

UL

1980 1990 2000 2010'Outside'

1980 1990 2000 2010

0

100

200

300

400

Year

La

nd

ing

s (

To

nn

es)

54

2013 2014

2015

Figure 6. Brown crab landings (tonnes) by statistical rectangle between 2013 and 2015. Black circles represent landings into Scotland. Data are from Fisheries Management Database. Green circles represent landings into Republic of Ireland – data provided by the Irish Marine Institute.

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

70

400

700

1100

1300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

70

400

700

1100

1300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

70

400

700

1100

1300

55

2013 2014

2015

Figure 7. Velvet crab landings (tonnes) by statistical rectangle between 2013 and 2015. Black circles represent landings into Scotland. Data are from Fisheries Management Database.

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

56

2013 2014

2015

Figure 8. Lobster landings (tonnes) by statistical rectangle between 2013 and 2015. Black circles represent landings into Scotland. Data are from Fisheries Management Database.

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

-8 -6 -4 -2 0

56

58

60

E1 E2 E3 E4 E5 E6 E7 E8 E9

38

39

40

41

42

43

44

45

46

47

48

49

50

3

50

100

200

300

57

Figure 9. Brown crab males mean size in landings (grey circles), mean size of the largest

5% of individuals (black circles) and length at first capture (open circles) by assessment

area, 1981-2015. Reference points 0.9L∞ (dashed line), LF=M (dot-dashed line) and Lmat

(dotted line), respectively. A minimum of 50 individuals was used each year to calculate

mean sizes.

58

Figure 10. Brown crab females mean size in landings (grey circles), mean size of the

largest 5% of individuals (black circles) and length at first capture (open circles) by

assessment area, 1981-2015. Reference points 0.9L∞ (dashed line), LF=M (dot-dashed

line) and Lmat (dotted line), respectively. A minimum of 50 individuals was used each year

to calculate mean sizes.

59

Figure 11. Velvet crab males mean size in landings (grey circles), mean size of the largest




mean sizes.

60

Figure 12. Velvet crab females mean size in landings (grey circles), mean size of the

largest 5% of individuals (black circles) and length at first capture (open circles) by

assessment area, 1987-2015. Reference points 0.9L∞ (dashed line), LF=M (dot-dashed

line) and Lmat (dotted line), respectively. A minimum of 50 individuals was used each year

to calculate mean sizes.

61

Figure 13. Lobster males mean size in landings (grey circles), mean size of the largest 5%

of individuals (black circles) and length at first capture (open circles) by assessment area,

1981-2015. Reference points 0.9L∞ (dashed line), LF=M (dot-dashed line) and Lmat (dotted

line), respectively. A minimum of 50 individuals was used each year to calculate mean

sizes.

62

Figure 14. Lobster females mean size in landings (grey circles), mean size of the largest




mean sizes.

63

Figure 15. Brown crab sex ratio (percentage of males) in landings by assessment area,

1981-2015.

64

Figure 16. Velvet crab sex ratio (percentage of males) in landings by assessment area,

1987-2015.

65

Figure 17. Lobster sex ratio (percentage of males) in landings, 1981-2015.

66

Figure 18. Male brown crab fishing mortality (Fbar) time series for the last four

assessments in relation to FMSY. For the Shetland area, F values (on a different scale in

the right vertical axis) are shown as estimated using two sets of biological parameters

(Shetland and rest of Scotland).

67

Figure 19. Female brown crab fishing mortality (Fbar) time series for the last four




68

Figure 20. Male velvet crab fishing mortality (Fbar) time series for the last four




69

Figure 21. Female velvet crab fishing mortality (Fbar) time series for the last four




70

Figure 22. Male lobster fishing mortality (Fbar) time series for the last four assessments in

relation to FMSY.

71

Figure 23. Female lobster fishing mortality (Fbar) time series for the last four assessments

in relation to FMSY.

72

Figure 24. Male brown crab biomass and yield-per-recruit (YPR) predictions given

changes from current effort by assessment area, data from 2013-15.

73

Figure 25. Female brown crab biomass and yield-per-recruit (YPR) predictions given


74

Figure 26. Male velvet crab biomass and yield-per-recruit (YPR) predictions given changes

from current effort by assessment area, data from 2013-15.

75

Figure 27. Female velvet crab biomass and yield-per-recruit (YPR) predictions given


76

Figure 28. Male lobster biomass and yield-per-recruit (YPR) predictions given changes


77

Figure 29. Female lobster biomass and yield-per-recruit (YPR) predictions given changes


78

8. Annexes

8.1. Annex A: Sampling Data – Decisions on Which Species/Areas Stock

Assessments Were Run

Table A 1 – Decision Table for Brown Crab

Brown crab

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol

Assessed in 2009-12?

N individuals/landings sampled

N years available for average LF 2

Sampling seasonality (quarters) 2/3/4 2/3 2/4

LFD shape

Assessment 2013-15

Table A 2 – Decision Table for Velvet Crab

Velvet crab

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol



N years available for average LF

Sampling seasonality (quarters) 2/3/4 2/3/4 2/3/4 3/4

LFD shape

Assessment 2013-15

Table A 3 – Decision Table for Lobster

Lobster

Cly

de

East C

oast

Hebri

des

Malla

ig

Nort

h C

oast

Ork

ney

Pap

a B

ank

Shetla

nd

South

East

South

Min

ch

Sule

Ulla

po

ol



N years available for average LF

Sampling seasonality (quarters) 2/3/4 1/2 2/3/4

LFD shape

Assessment 2013-15

79

Table A 4 – Legend for Decision Tables


POOR: No sampling or very few animals sampled (average <

100 per year)

OK: Few animals sampled (average < 500 per year)

GOOD: Several animals sampled (average > 500 per year)

N years available for average LFD

POOR: < 2 years

OK: 2 years

GOOD: 3 years

Sampling seasonality

POOR: Less than two quarters sampled over the 3 year

period

OK: Two or three quarters sampled over the 3 year period

GOOD: All quarters sampled over the 3 year period

LF shape

POOR: No data or very few animals sampled

OK: LF with some spikes

GOOD: Approximately normal with no spikes

80

8.2. Annex B: Length Frequency Distributions

Figure B 1. Brown crab carapace width (mm) frequency histogram by assessment area

averaged over the period 2013-15. The data presented are aggregated by 5 mm

increments and shown as a proportion of the total landings.

81

Figure B 2. Velvet crab carapace width (mm) frequency histogram by assessment area

averaged over the period 2013-15 for males and females. The data presented are

aggregated by 3 mm increments and shown as a proportion of the total landings.

82

Figure B 3. Lobster carapace length (mm) frequency histogram by assessment area

averaged over the period 2013-15 for males, females (berried also shown). The data

presented are aggregated by 5 mm increments and shown as a proportion of the total

landings.

83

8.3. Annex C: Brown crab and Velvet Crab per Recruit Analysis Using Different

Biological Parameters

Figure C 1. Male brown crab biomass and yield-per-recruit (YPR) predictions given

changes from current effort by assessment area, data from 2013-15. The YPR

analysis was run using two different sets of biological parameters (Shetland and rest

of Scotland).

84

Figure C 2. Female brown crab biomass and yield-per-recruit (YPR) predictions

given changes from current effort by assessment area, data from 2013-15. The

YPR analysis was run using two different sets of biological parameters (Shetland

and rest of Scotland).

85

Figure C 3. Male velvet crab biomass and yield-per-recruit (YPR) predictions given

changes from current effort by assessment area, data from 2013-15. The YPR

analysis was run using two different sets of biological parameters (Shetland and

rest of Scotland).

86

Figure C 4. Female velvet crab biomass and yield-per-recruit (YPR) predictions

given changes from current effort by assessment area, data from 2013-15. The

YPR analysis was run using two different sets of biological parameters (Shetland

and rest of Scotland).

87

8.4. Annex D: Assessment Areas Names and Abbreviations

Table D 1 – Crab and lobster fishery assessment areas and abbreviations

Assessment

area Abbreviation

Clyde CL

East Coast EC

Hebrides HE

Mallaig MA

North Coast NC

Orkney OR

Papa PA

South East SE

Shetland SH

South Minch SM

Sule SU

Ullapool UL

Documents

Crab and Lobster Fisheries in Scotland: Results of Stock ... 0814_0.pdf · inappropriate. Brown crab tagging studies are currently underway in Orkney and these may shed light on potential